Cardiac rhythm management devices are implantable devices that provide electrical stimulation to selected chambers of the heart in order to treat disorders of cardiac rhythm. A pacemaker, for example, is a cardiac rhythm management device that paces the heart with timed pacing pulses. (As the term is used herein, a pacemaker is any cardiac rhythm management device with a pacing functionality regardless of any other functions it may perform such as cardioversion or defibrillation.)
The most common condition for which pacemakers are used is in the treatment of bradycardia, where the ventricular rate is too slow. Atrio-ventricular conduction defects (i.e., AV block) that are permanent or intermittent and sick sinus syndrome represent the most common causes of bradycardia for which permanent pacing may be indicated. If functioning properly, the pacemaker makes up for the heart's inability to pace itself at an appropriate rhythm in order to meet metabolic demand by enforcing a minimum heart rate.
Pacing therapy can also be used in the treatment of congestive heart failure (CHF), which is a clinical syndrome in which an abnormality of cardiac function causes cardiac output to fall below a level adequate to meet metabolic demand. CHF can be due to a variety of etiologies with that due to ischemic heart disease being the most common. Some CHF patients suffer from some degree of AV block or are chronotropically deficient such that their cardiac output can be improved with conventional bradycardia pacing. It has also been shown, however, that some CHF patients suffer from intraventricular and/or interventricular conduction defects (e.g., bundle branch blocks) such that their cardiac outputs can be increased by improving the synchronization of right and left ventricular contractions with electrical stimulation. Cardiac rhythm management devices have therefore been developed which provide electrical stimulation to one or both ventricles in an attempt to improve the coordination of ventricular contractions, termed ventricular resynchronization therapy. Such electrical stimulation will be referred to herein as “pacing” even if the stimulation is not delivered so as to enforce a particular heart rate.